Yes you should be able to trust your vet but as soon as the vet said use Beaphar, I would have been out of there! That's clearly not a knowledgeable vet!
We have told you that Beaphar is more for prevention (which we don’t recommend doing anyway due to the risk of resistance forming) so isn’t effective at treating an active case, shampooing isnt effective and trying to use these products will not make this problem go away, therefore there is plenty of harm in trying them because your piggy is not receiving the treatment it needs. If you try to shampoo your piggy for a problem you dont even know they have, then all you will be doing is upsetting the skin's natural microbiome and causing even more problems on already irritated skin and further delaying getting the correct treatment. It may not cost as much and that is a bad thing, its not a bonus - using such methods to attempt to treat an illness will cost more in the long run as you will still need to go and get the correct treatment but by that point, you will have delayed getting the right treatment for so long, that you could end up needing even longer courses of the correct treatment to solve the problem, plus potentially risks secondary health issues.
The issue your piggy has with the skin is a huge problem and its just as important to get vet care for it just as much as a problem requiring antibiotics or pain meds.
Please do use the
Recommended Guinea Pig Vets to try to find a better vet. All of the vets on here are member recommended for being cavy savvy.