yep, not illegal in the slightest.
A lot of people like to get into it by buying a sniper, because that's whats fun on video games. However, as said before a sniper rifle is weird with airsoft, if you want a good one you need to upgrade stuff no matter what.
Normally everyone would recommend an AEG me included, but I can see how they are fun. However to get them to a decent level, a lot of money is needed for upgrades.
TBH, it could be dangerous, but I have played with a mate at 500 fps sniper in <20 metres and it was ok for me however if you know its that hot, you definitely get scared, I would never do it again imo. Even then many things can go wrong, and it could potentially cause problems.
Overall, I would tell him to think long and hard about his decision. AEG's need little to no maintenance and are very effective. Maybe you should try some out before hand to see if he likes it or not.
IF he needs a sniper, maybe look at the alternative which could be the Tokyo marui VSR10. Basically, this is THE SNIPER to have for anyone wanting to get a great sniper rifle OOTB, however it shoots at 300-330 fps and so normally people upgrade them, spending 500+ quid on upgrades for it so it shoots 500 fps and is very accurate. However it is good ootb, and if you are worried people could get hurt it is less powerful, but for backyard shooting thats fine...
hope this helps