I am in a graduate program. In our first year, we were put into small groups to work on a project. I befriended one of my fellow classmates who I will call Frieda. Frieda and I usually hung out with another friend of ours. We were part of a research group later on last year. However, Frieda started acting a little strangely. She stopped hanging out with our other friend, with whom she was close. She started working a lot rather than spending time on classwork, and she cancelled on plans about 90 percent of the time. At that point, I decided it would be best for me to focus on my other friends and just enjoy Frieda when she was present.
However, Frieda has started to email me about dinner plans. We had plans for a week ago, but she decided to go home early from work. Then she asked to reschedule. Twice.
I'm getting very tired of making plans with Frieda unless it's in a group situation where the plan won't be harmed if one person doesn't show. I don't want to keep rescheduling one-on-one dinners with her because I'm very tired of her canceling at the last minute. It's also pricey to eat out regularly, and she has never attended a dinner at my house (party or not). However, we're going to be around each other a lot for the next few years, and I don't want to handle this situation poorly.
When Frieda asks about dinner again, what's an etiquette-approved response?