This is my experience from the perspective of the mother:
It would probably help a lot if she told her mother that she has a hard time right now.
Wanting to keep this a secret is understandable, but might be making the situation only worse. For us "normal" people there's no reason to avoid the topic of relationships. On the contrary, it's actually a rather common smalltalk topic among family and friends.
Step 1: Tell them to avoid a topic
If you need someone to avoid an otherwise common topic, you at least need to tell them to avoid it. I have several people in my family and circle of friends that are or were dealing with times of depression, anxiety, burnout and other physical and medical conditions. I saw them change and struggle sometimes, but since I wasn't affected by the same problems, I kept on living my life as usual. I didn't know how to properly help and support them and my few attempts at well-meant advice didn't help at all or backfired.
It took some honest conversations, that were sometimes emotionally draining for both of us, to change my behavior in a way that they felt more comfortable and supported by me. Without these conversations, I francly had no chance of finding the "right" behavior on my own, because I simply cannot understand how something insignificant to me affects them so much. To this day, more than 10 years after the first diagnosis in my family, some of these rules feel artificial and outright ridicilous to me. I can remember, accept and follow them, but I will never be able to truely understand them because I cannot read the thoughts of the affected person and don't feel what they feel.
The mother of your friend cannot understand why your friend doesn't want to talk about relationships. The only chance to make her avoid the topic is to outright tell her, in no uncertain terms, to avoid it.
Step 2: Tell them why
I assume this is a big problem for your friend.
Her mother is not aware of the problem, and my friend would like it to stay this way - she believes her mother would press her for all the details (not taking "no" for an answer), talk to other members of family about it, etc.
When she tells her mother to not talk about a certain topic, the mother will certainly ask "why?". I see 2 possible ways to deal with it:
- Give her a vague answer and refuse any detailed information
- Be honest and tell her about the severity of her condition
Solution 1 could probably best be achieved by preparing one answer utilizing an I-message, like:
I need you to not talk to me about boyfriends and relationships because it hurts me too much to talk about it.
If the mother digs for more information, repeat the same answer over and over
Because I'm having a really hard time right now and talking about it hurts too much.
It just hurts. I don't want to talk about it.
In my experience, solution 2 might be better in the long run. Being in medical treatment is more severe than "a little heartbreak" and an objective proof that there is something wrong that's outside her control.
If your friend stubbed her toe and dind't want to walk, her mother would probably tell her to stop making a fuss. If her foot were in a cast because the toe was broken, the mother probably wouldn't be so insensitive. The attending doctor is something of an outside authority - they wouldn't be treating a stubbed toe or a "little heartbreak", so the fact that they are treating your friend means that her condition is severe enough to warrant professional attention.
In my family we were mostly open and supportive when discussing mental health problems and I feel like admitting and discussing their own issues actually helped affected people comming to terms with those issues. Of course, your friend knows her mother best and whether or not she would be supportive or rather gossip.