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

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.
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.