Here is a link that shows current backups at all the border crossings in the area:
http://www.borderlineups.com/Usually the lines are longest at the Peace Arch crossing. I have never had a backup at the Sumas, WA/Abbottsford, BC crossing, but that may be out of your way. I've heard the Pacific Highway is a good way to avoid delays, but I've never used that one myself.
I haven't crossed by land there in a long time, so it's hard for me to give you an idea of how long the waits might be now. Last time I went home (my parents live in Bellingham, which is 20 miles south of the Peace Arch crossing), it was Thanksgiving weekend, the Canadian dollar had just surpassed the US dollar, and the lines were running about 3 to 4 hours at all crossings, so we just flew into SeaTac and did not go to BC at all. Right now, it looks like there is no delay at the Peace Arch and a 20 minute wait to cross at Sumas. I would guess that on a weekend there would be more of a wait, although I don't know how much. Back when I used to cross regularly, there was never a backup at Sumas, and at the Peace Arch, it was usually less than an hour.
If he is catching a plane at Sea Tac, you also need to take into account the possibility of a bad traffic back up going through Seattle. I'd actually suggest going down the night before his flight and staying over near the airport.