I actually registered for the 5K 4th Feb this year - didn't realise it was quite so late. Very definitely not cheap (makes the marathon look like a bargain) but I'll probably do it again next year. I don't know if it is the same every year but your partner might want to know that the tech shirts from the 5K seem to be men's sizes as opposed to the marathon where, if you are female and go for small you will get female small sizes.
Buses start at 6AM but there will be a scheduled time for your wave. You will receive (detailed ) instructions with your race passport.
Security considerations means you can only leave a (limited) amount of baggage in a clear bag at Boston Common. Anything you take on the bus needs to be used / worn / eaten or thrown away.
Why I like this one? - As a race it is great as long as you can cope with a few undulations. From a running perspective it's less crowded than London, less street furniture etc. to trip you up on route. Water and Gatorade every couple of miles in paper cups rather than bottles.
What makes it different? Boston isn't a huge city like London but I've heard that Dublin is the most similar where the whole town is involved. Marathon Monday is their local public holiday. Families with houses along the route sit out, offer food and drink, have barbeques. Bars open at 8AM for supporters. For the major part of the route there are no metal barriers, only a bit of rope.Fantastic atmosphere.
Last year we also entered the 5K race on the Saturday. Very pricey but a nice experience, fast course with tech shirt and medal.
Can't give you any ideas on how to run the race but be prepared for any weather. In the past I've been sunburned, last year the temperature remained in single figures and it didn't stop raining - I'm very glad I went to a charity shop to buy throwaway extra layers.
My main recommendation is embrace it. Go to the events, wear the gear, drink the beer.
I'm not overly keen on road marathons but I love this one and there's definite PB potential.