There is another option for the IOM, in August they hold the Manx Grand Prix. Granted the big names might not be there, but it is still a great few days with some full on road racing. Here is a link to the website
http://www.manxgrandprix.org/MGP/About-the-MGPAccommodation tends to be cheaper and easier to find.
The ferry from the mainland is always hugely expensive and flying from Ireland or England can be cost effective if you can sort transport on the island out. There are lots of places to see on the island and quite a bit to do.
Seafood down in Peel. Peel Castle. (Have you seen the little microcar the Peel P50? It comes from the Island.)
Lots of wildlife viewing including Basking Sharks, I believe August is a good time of year. The Sound is a great viewing area for dolphins etc.
There is a couple of motor museums on the Island, one of them I think has a Bonneville bike on show.
There are lots of campsites on the island, and the number probably increases 10 fold during TT week ( by farmers opening up fields so "facilities" can vary widely), but it does get wet over there. Bring lots of clothing changes, it really does get wet!
There are some fantastic pubs away from the bigger towns and some stunning scenery all around especially in the southern part and behind the mountain. Just go out and explore.
During TT week, and the MGP week whilst practice and racing is on a lot of roads are closed and it can mean travelling around the island can get difficult for a couple of hours each day.
It is a great little island and your friend will have a great ( if a little wet) time. We were there in 2007 and it rained, rained and rained again.But I still had a blast and rode my sidecar all around the place.