Oh, no, don't phone them up.
Send a nice get well card:
Dear parents of PTM, we are so sorry to hear you are both unwell, and send you our best wishes. As you know, PTM and I are no longer together, but I hope we can keep in touch, and I'm sure you're aware how much we all care.
I'm sorry I haven't written earlier, but I only found out about your ailments recently, via a letter from PTM to the solicitor, in which he says he is spending a lot of time with you. I can reassure you that I and your lovely grandaughter are absolutely fine, and I'm sure you are reassured by your frequent contact with PTM, that he and his new lady (have you met her yet? I'm sure she is everything he deserves), are well too.
As he made it clear to the solicitor that he is spending only a small amount of time at his current address, and a lot of time tending to you both, I'm sure you wouldn't mind if your address was used for further legal correspondence. It is in all our interests for things to move ahead as amicably and smoothly as possible.
Do give me a call if you feel up to it; obviously I would love to visit, but I'm sure you understand that I'm rather busy with my new social life, job, and of course there is always the lovely family to bring up! Do keep in touch.
Love
Wisey XX