If he has a mortgage then there is absolutely zero point putting any away into an ISA. ISA rates aren't even above inflation right now, so everyone with money in one is losing money
If it's an instant access cash ISA, sure. If it's S&S and not holding the same geriatric coffee table ******** most people go for (Miton Special Sits, Invesco Perpetual High Income, Blackrock Gold & General etc), the tax free returns can be quite attractive provided the returns take you over the CGT allowance.