Friday looks like your best bet.
Sunrise in Fort William right now is about 7am, and sunset is about 7.30pm. Getting up and down the Ben on Saturday and then back to FW for your bus would be a stretch unless you started in the dark - and as inexperienced walkers I would recommend against that. There may also be a temptation to rush the descent: many accidents happen during descents at the end of the day when people are tired. So I'd tend to exclude Saturday.
If you arrive at 9:55 on Friday and your friend is already there then you should be able to nip into Morrisons and be on the bus/taxi to Glen Nevis by say 10:30 to arrive at the YHA about ten minutes later. That gives you about eight hours to get up and down, with no hard deadline at the end of the day.
(There are lockers and showers at the train station, and the bus station is adjacent.)
Since you said you're inexperienced:
* Bear in mind that eight hours may still not be enough - it depends on your level of fitness. Remember that when you get to the top you're only half way, and be prepared to turn back. The mountain will still be there next time.
* Don't let its proximity to FW deceive you: Ben Nevis is a serious mountain and the weather can be nasty at any time of the year. In particular, navigating back back across the plateau from the summit to the correct descent path can be tricky in poor visibility and requires map/compass skills. See
http://www.mcofs.org.uk/ben-nevis-navigation.asp for info.
I hope you have a great walk!