Words for a real moment
How to support someone who is caring for another person
People doing a lot of caring are often offered vague help. One bounded practical offer is easier to receive.

Words you can use
A ready-to-send version
I see how much you are holding together. Would a short call, a voice note, or simply a quiet message suit you? You do not need to explain anything, and no reply is required. If today is not right, that is completely fine.
A useful approach
Recognise that they are carrying a lot without diagnosing burnout or asking for private details.
Give them a choice between a brief chat and a message they can answer later.
If they decline or stay quiet, check once tomorrow and then give them space.
Make it fit your person
Change the relationship, timing and tone
No name is needed. Optional details are not stored.