Primary issue:
Is this Mums PPR - main residence? If yes, then no CGT applies. If no, then CGT will apply.
If this is only gift/inheritance then probably no CAT.
Stamp duty will be half of 6% is he is a FTB.
On her death, if they hold as joint tenants, the property will automatically pass to him. At that stage, there may be CAT implications but no stamp duty or CGT.
Like most situations, he should get proper advice. If you are going to be involved and possibly paying money into the house, you should consider your own position.
mf
mf