I don't go there regularly but I have a few clients in the Law Library and parking is a pain. Lots of streets with clearways, loads of back streets, one way streets and not a huge amount of parking. It's not cheap, so you wouldn't park there for the day. Public transport is a much better option.
The ParkRite multistorey nearby (backing onto Smithfield) does a student daily parking rate as far as I remember. If he can avail of that it might be a good option?
I agree with @SBarrett that the on-street parking is limited and would be expensive for full days
In the Law Society-Blackhall place- is free though, surely? At least whenever I attend for a CPD it is. If I am going to fourcourts/mediation I would park in the parkrite as nearer.