Boyfriend Texting Girl

So my boyfriend has a very wide circle of friends, and he's kept in touch with a lot of people from his college days, including this woman that we'll call Kimberly (not her real name). My boyfriend and Kimberly have never been an item, but they've hung out on occasion in the past. Well, recently Kimberly has been texting my boyfriend a lot more often. She's been telling him all about her dating difficulties and how she's single and the man she wants isn't interested, etc. My boyfriend has been completely transparent about the whole thing, but he's also been very supportive of his friend. I trust my boyfriend, but I don't necessarily trust Kimberly. She's been talking to him way more than just the occasional update; these are lengthy text conversations. However, my boyfriend has never tried to place restrictions on me, so it would be unfair of me to tell him I'm not comfortable with him speaking to Kimberly anymore. What should I do?