What would the tax treatment be? Would this be an allowable expense for the business?
No, but he can gift aid it for some tax relief if he's higher rate.
Yeah, Gift Aid's the route. He needs to decide whether that's 10% gross, which will be 8% net, or 10% net, which will be 12½% gross.
They're his own parameters so very much up to him.
He won't get relief against Class 4 NI through Gift Aid.
In theory, he could get relief as an expense but he'd have to show that the donation was wholly and exclusively for the purposes of the trade, which would be nigh on impossible, I would've thought.
Are your clients which don’t make such donations unethical?
I would recommend you advise your client against this, not for any particular tax reasons but from the inevitable PR disaster that it turns into when his definition of profits or donation is disputed.
What's it to do with PR? Some people simply want to give (back) and do so quietly.
Which, OP, is why there's no business deduction - it's a personal matter.