Friend said she woke up last night having trouble with swollen tongue. what could have caused that? second time in a month.

Food allergy: Occasional episodes like this make me think of occasional exposure to a food your friend is allergic to or some ingredient in the food. Simplest first step is to do a food diary and record everything that she eats over the next month. Then you can go back after a reaction and see if you can find similarities. Another method is with igg food allergen testing through a specialty lab like metametrix.

Allergic reaction?: This could be caused by some type of allergic reaction or even something systemic. Is there a history of this occurring? Is there any problem breathing? If so, she should seek immediate attention. She could be having a reaction from something she ate or drank prior to bed. Does she have any vitamin deficiencies? Regardless, this should be checked out and investigated. Start with her dentist.