Can you impose a time limit on a plumber to do remedial work?

[lazarusb]

My ds's partner has had some very shoddy work done by a plumber. Unfortunately she paid him upfront. There is water leaking close to electrics and various other issues with badly fitted wastepipes, tiles etc.

He initially said he'd give her the money back to get someone else to fix it, then reneged on that and said he would do it, but only when it was convenient for him and said it could be months!

Can she enforce a time limit or payment (her preferred option)? He has accepted that the work is faulty although most of this discourse has taken place via FB.

Thank you in advance!