I have no idea what to do

My fiancé mum is 52 & been dating a 28 year old for 5 months, about a month ago she told me he occasionally has coke, each to their own I guess... last month she was trying to get a loan... so my fiancé offered to borrow her £1,000. I wasn’t happy about it, but it’s his mum.... today I’ve been texting her and she was saying they might be splitting up, so i went round to see if she was ok. She had a break down & told me he’s addicted to heroine & he’s been spending £60 a day, the money she borrowed he took £400 of it and used it for drugs, apparently he’s been and getting help, but the person he’s been doing it with is family & he’s always up there. She’s now having to hide her bank cards & valuables incase he takes them, he’s even said he will ask my fiancé for money cause they get on. She’s already lent his over £3,000 so he can get his drugs

I promised I wouldn’t say anything. But I can’t lie to my fiancé, but I want him to know want his mum going through, but I don’t want her to fall out with me, I’m worried cause I don’t know what length she will go to, to get money when he needs his drugs, she’s already lied to her son. Do I just let them get on with it??