I am sorry to say it but her behaviour is weak, self-indulgent and, for a woman in her thirties, she needs to be far more self-reliant.
I say that from the perspective of someone who lost a parent in my mid-late twenties and who had to hold down a demanding job (with significant responsibility for the welfare of others) immediately afterwards. I had a very difficult time and sometimes my grief would rise to the surface, but largely I just had to get through it. A few years later I had counselling, which was very helpful.
I certainly wasn’t splurging on someone else ten times a day because a) who would have time for it and b) the technology didn’t exist. I was already married, which was a huge support, but even my DH wouldn’t have wanted to hear from me constantly about how I was feeling!
My strongest piece of advice to you is to de-install WhatsApp. Explain to your son and partner what you are doing and why. The chat-based messaging technology is far too immediate, easy to use and intrusive on the recipient. Make her contact you via text, email or another messaging app.
Every time she sends you a message that is just self-pity, send back a sympathetic but slightly formulaic response:
‘I am so sorry to hear that you are feeling like this. I am sure that things are better than they seem. Can you book a session with a counsellor?’
Leave far longer gaps between messages - turn off any notifications - and only respond to genuine communication.
You need to break this pattern, for both your sakes.