Magicless group?

[Painfully]

If it makes sense, why not keep the sorcerer as part of the group for now. Keep him as an NPC and simply keep playing similarly to how he has behaved in the past. When it becomes convenient for the DM, from a storyline POV, have the sorcerer leave the group and the other players can decide what to do next.

Leave the choice up to the players, but don't let them go cold turkey if they are mid-adventure just because the player is leaving.

 

[Saeviomagy]

As it's written, your party should:
1) Kill things really quick.
2) Survive pretty well
3) Need much much longer to recuperate after they decide to stop plowing on
4) Be able to scout quite well
5) Have problems with truly unusual situations (unless you've let your players buy invisibility purge/see invisible/detect magic/fly items, they could be totally and utterly wiped out in a single fight, with almost no way to fight back)

Basically the only change I would make would be to allow your players to buy those items if they go looking for them.

 

[Conaill]

They should be smart enough to realize that a non-magic party in a magical world is in for a lot of hurt. If so, they can easily hire someone to accompany them. A combat-worth cleric sounds like a good choice.

Alternatively, if they're getting close to 6th level, they may wish to seriously consider picking up the Leadership feat.