Hi,
My suggestion is, have them create new subscription in their tenant, next transfer VM, disk(s), Public IP, VNet, NIC, etc. to this new subscription. Once transfer is successful, have them transfer billing ownership of this new subscription over to you.
Specific steps to transfer ownership varies based on type of subscription/scenario.
What I would recommend is to have them open new support request and provide the engineer with precise details of their subscription as well as yours in terms of offer type. They should explain that they are wanting to transfer ownership of the new subscription to you.
Article below has details on transferring ownership of Pay-As-You-Go subscription and if you look on left there are links to other types.
https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/billing-subscription-transfer
Please click Accept Answer and upvote if the above was helpful.
Thanks.
-TP