My mother in law just had some dental work done and now she has a fever blister on her lip. does this mean she has hsv-1 and is contagious?

Perhaps: If she had a previous history of hsv, then the stress of dental work can lead to a localized breakout. If it is hsv, then yes, she is contagious. Anytime there are active hsv lesions - oral or genital - then one is contagious. It's the fluid within the blister that contains the virus.

Could be!: Any physical trauma can predispose it but i believe it could be from the anesthesia effect and her unknowingly biting on her food and part of the inner buccal mucosa causing the " fever blister" lesion! see your dentist for help! regards!