My girlfriend has bumps on the back of her tongue. Her allergy dr said they're from post nasal drip. Any way to get rid of them when they flare up?

Treat postnatal drip. By controlling the postnasal drip with appropriate meds (e.g. Intranasal steroid spray and/or intranasal antihistamine spray, oral antihistamines) the bumps should resolve. There is no therapy which can treat the tongue directly.
Sounds wrong to me. 1) post-nasal drip will not cause bumps or swelling of the tongue. It is just mucus, and even infected mucus won't do this. 2) tongues normally have bumps - papllae, and in the posterior of every tongue there are larger papillae. This is normal. You many or may not see these every time you look depending how far out you stick your tongue.