So here’s the scenario. I have a player who is a beast master ranger (knowing that they tend to be a bit weak).
In general he wants to use his wolf to stand sentry most of the time, “use the help action” to give him advantage on perception checks (which then translates to a +5 to passive perception).
he already has a very high passive perception (observant) so this would definately push him into the “very high” territory.
what do you think, is that a reasonable way to use an animal companion?
I rather see a problem with the Observant feat than with letting the wolf pet help...
Normally, using passive perception means to Take 10, and it is meant to represent the average result of a task done repeatedly. It can cause a DM some problem when setting the DC of hidden things because then a passive check means to compare two static numbers (not so much with hiding creatures who can always roll their stealth check, maintaining some randomness), and this can lead to the situation where the DM is implicitly deciding in advance everything that will or won't be found. If it had been for me, I would have completely avoided passive checks when designing the 5e ruleset. But there are still a couple of aces in the DM's sleeves: first of all even a passive check is still subject to rule zero, meaning that the DM can always choose you don't get to make the check at all (whether it is resolved with an active roll of the dice, or using a static number) if the circumstances aren't right; second, because a passive check represent a task done repeatedly, the DM doesn't (and probably shouldn't) grant an active check when the PC "misses" the passive check, at least not unless the player actively asks for it.
[perhaps a bit more explanation on the latter part: if you tell me your PC will "keep looking for hidden stuff all the time", I will use your passive perception score but I will NOT jump up and grant you an active Perception check on my own initiative when there is indeed something to find and you missed it with your passive score! It will have to be you telling me that you think there is something to find here and now and want to actively stop and search better than your "keep looking". I interpret that routine task of a passive score as "going through the motion", meaning doing an average job at best]
Add the advantage from the pet aiding the Ranger (or whatever other source of advantage) and passive perception becomes Take 15. This is still not a problem for me, you're not automatically getting your best result all the time, and there is still a reason for a player to take the initiative of stopping somewhere and searching better with an active perception check.
Note that in general I don't have much problems with a very high chance of avoiding surprise, after all there are other abilities in the game (like the Alert feat) which make you completely immune to surprise! But I do have issues with noticing hidden objects, traps, secret doors, hazards... everything that itself is passive doesn't work well with a character being passive in looking for it.
However, enter the Observant feat, and I feel we're in Houston... because the damn feat grants you a +5 bonus ONLY on passive scores, and not in the form of advantage, so it will stack with it, and make using passive perception the equivalent of Take 20. And I do not like this at all, because then it means the PC will get the best result every single time. There is also no more reason to ever asking for an active roll, meaning that a player is not encouraged to think in-character and guess where there might be something hidden, it can be completely on auto-pilot. If the Observant feat had granted advantage instead of flat +5, it would not have stacked and it would still be Take 15. If the Observant feat had granted +5 on ANY perception check (not only passive perception) then it would have also been ok for me because an active roll would have still been beneficial, again passive would have been still Take 15.
I still have the first ace in my hand, rule zero allowing me not to always grant you even the passive check, I can definitely rule for example that a pet simply has no capacity in aiding you to find a hidden door or object (a concept it may even struggle to grasp in the first place) unless it smells interesting, but it feels a bit sour to use rule zero often... it feels like now I have to punish the player where the guilty is whoever designed the Observant feat without realizing this possible situation. To get out of this mess, if I had this situation in my group, I would tell the player to swap the Observant feat with something else if they want me to allow the pet helping all the time.