If it were me, I would do a 100% water change and clean out the gravel to get rid of anything that has fallen into it like poop, uneatten food. ect and stop the bettafix.
NEVER TRUST PETSMART OR PETCO EMPLOYEES - THAT IS RULE No. 1
does he look like he is covered in salt?
like this- if so, you need to treat for ich which is a parasite and that would need to be treated with some kind of parasite medicine like Quick Cure.
However, I think one of his problem is the tank is too small for two fish. Ammonia will build up quickly and the only way to get rid of it is by water changes every week. He may also be cold, like others have suggested and/or stressed out by the female betta. The 2.5 minibow is a nice tank but for one fish, not two. try removing the female into something else (a storage container will work if its at least 2 gallons-just make sure its covered as females like to jump. Saran wrap works well but be sure to poke holes in it), do a 100% water change and see if it males ANY difference at all. As soon as you can get a heater - that will be great too. 25 watt adjustable ones are much better then say the tetra preset ones but if that's all you can find, its better then nothing.
If you want to keep them together, you can divide up a 5 or 10 gallon tank. That will give them enough of their own space and you can cycle a 5 gallon tank (if you do not know about the nitrogen cycle and you choose to go with the 5 Gallon and use a filter - please read up on it).
Yes, they are more hardy then most fish but one still needs to keep their water clean. Sometimes I want to strangle petstore employees for their pure...stupidity. Unless you plan on upgrading to a 5 gallon or bigger and cycling the tank, you don't really need the water test kits. HOWEVER, if you can get the ammonia test kit (liquid one not the strips) I think its a good idea. That way you know exactly where you stand with regards to the ammonia level.
either take a close up pic of him or shine a flashlight on him. What you are looking for is anything that looks like gold specks or gold flakes. Velvet is another parasite but normally you can only see it by shining a flashlight on the fish or taking a flash picture. Velvet can also cause the symptoms you are seeing.