Every time a DM let's the players encounter a lycanthrope, there is a possibility that the character gets infected and turns into a lycanthrope himself.
I'd say this effect would be wellknown from stories and battlereports, or at least should be.
After a battle with lycanthropes, a party should take the necessary steps to remove the curse, for the initial fases of lycanthrope bring more trouble than it's worth.
Remove disease or heal cures lycanthrope if the caster is level 12+. Remove curse and break enchantment cure lycanthrope regardless of level.
Even a 5th-level cleric can cast remove curse. Any midlevel town should be able to help the party out here. Untill the character is cured, the party is travelling with a timebomb.
The first night of a full moon, the character changes into animal form (in this case the form of a dire rat). In this form, he'll strike out at anything he comes across.
In this form he can be a danger to the party, as he gains damagereduction and improved stats during the change, but doesn't lose his hitpoints and continues using his bab.
Unless the party has taken precautions against the inflicted rampaging around, like locking him up, they're going to have to deal with the dire rat.
If and when the character changes back, he'll be able to make a check the next time he might get an unvoluntary change (during another full moon, or when taking considerable damage). He could also try a voluntary change.
I'd rule that any unvoluntary change would turn the character inflicted into an npc for the duration of the change. The rest of the party might be resentfull if the player picked up the dice against them. The werecreature would have no knowledge of friend or foe, and lash out at everything. The party just lost an ally and gained a hazard. A character that had gone through a voluntary change would remain in the hands of the player though.
The party could be interested in playing with an inflicted character though. I think this should be a group-decission. If the rest of the group doesn't want to adventure with a timebomb, they shouldn't be forced to. But if they would be willing to try, it can make for interesting roleplaying. The party would have to take regular full moon preparations. If the party would find themselves in town, they might have to convince the cityguard to let them use one of their holdingcells for the night.
If all players agree to let the inflicted stay a lycanthrope, you shouldn't let the pc level untill he's gathered enough xp to account for the ECL of the lycanthrope type. That way he won't be overbalanced when he voluntary takes lycanthrope form during combat.
The player will have to be wary of the "Bruce Banner" effect though. When he can't use his alternate form in combat, he's much less effective than the other characters.