Blocking simply hides people from each other, no-one else. So mutual friends can see everything each person writes, but they can't see each other. Two people who have blocked each other can comment on the same thread, and be part of the same conversation, but they may not realise it. Also, blocking does not affect past content - anything that is there already will stay, unless it's on a person's own wall and they specifically remove it. However, if your DS blocks his ex, she will not be able to look him up or see anything he posts or writes from now on.
Deleting an account does not delete any past comments or tags. In some ways it gives a person less control, because although they can't be tagged any more, they can no longer remove past tags.
A better option might be to disable his account. This has the same effect as deleting it, but is easier to do, and isn't permanent.
There is nothing your DS can do about his ex-girlfriend's FB content. If she writes about him or posts pics, there is nothing he can do - exactly like he couldn't stop her keeping a diary, going through boxes of old photos of him, or talking about him to her friends.
One question for your DS to consider is whether this girl is so important that she's worth losing all his other social FB contacts? She's going to continue to influence his life, if he misses out on social chat, invitations to parties, etc... The end of a relationship is painful and difficult, and deleting his FB account is part of wanting it all to stop, but it's almost certainly not worth it in the long run.