If it is him doing his own DIY for his use, and not as a paid service for someone else, then it is legal for him to do it.
He cannot however do any gas installation for anyone else without certification.
That is not to say however, that it is sensible to do your own unless you are trained. There are many things that could go wrong ad gas is a dangerous thing.
Worth him bearing in mind, that if there happened to be a problem as a result of the work, which causes damage to his and/or any other proprty, his insurance would be highly unlikely to cover him having done the work himself. Also i think he will find that noone will enter into a servicing contract on the boiler if he fitted it himself, so he is effectively signing up for a lifetime of home maintenance or expensive call out charges.
He may also like to think what will happen if he ever wants to sell. If it becomes apparent it was a DIY job, it may well put buyers off.