Hi, Kristen,
First of all, the trains from London to Glasgow depart from Euston, and terminate at Glasgow Central. The commuter trains to Balloch leave from Queen Street station, so your friends will have to make their way from one station to the other. There is a taxi stand right outside Central. There is also a free shuttle bus between the two stations. They only need to show the driver their train tickets. The bus runs very frequently. It's about a 15 minute walk, but your friends may not be up for that after a transAtlantic flight and a five hour train ride. Especially if they have a lot of luggage.
Secondly, the trains to Oban depart from Queen Street as well. So your friends should get a ticket from London Euston to Glasgow, the cheapest way possible, make their way across to Queen Street, then buy separate round trip tickets from Queen Street to Balloch. They won't need to reserve ahead, as this service is operated by ScotRail, and it's a commuter train, so it's rarely full. If it is, there will be seats available after Dumbarton.
Once they get to Balloch, they may have to rely on the good graces of the B&B owners for a lift to their accommodation. Many B&B owners are happy to provide this service, especially if your friends are staying for more than one night. If not, they may have to disembark at Alexandria, as taxi service is woefully lacking at Balloch station.
Once your friends have spent time at Loch Lomond, they can return from Balloch to Queen Street, and catch the train for Oban. There are four trains daily that would fit nicely in to their schedule. They can get round trip tickets from Glasgow to Oban, again, the cheapest possible. If they are not returning to Glasgow after Oban, then they'd only need a one way ticket. I think that there's something called "Two Together," which gives train going travelers a discount. I'm sure that others on this forum will chip in with more information.
If they get in to Glasgow too late in the day to get to Balloch (There are trains every half hour - the last one leaves around 11:00 p.m.), they could stay overnight in central Glasgow, and catch an early train the following day. The trip from Glasgow to Balloch is just under an hour.
As far as cost goes, the cheapest round trip fare I've found is 160 pounds from London to Glasgow. That's on Virgin Trains. They would have to estimate what time they'd be getting to Euston, in order to book a specific train. That's the off-peak fare. If they miss that train, and have to take a later off-peak train, it's an extra 10 pounds each. However, if they end up having to catch a peak train, it's 10 pounds plus the difference in fares. The fares go up exponentially from off-peak to peak.
If they don't mind 11 hours on the road, Megabus goes from London to Buchanan Street Glasgow for 10 pounds 10 pence. I've done that route. It's no fun! :( I even went from London to Aberdeen on the bus one year. 14 hours of pure torture!
Hope that helps!
Best wishes!
Mike (Auchterless)
p.s.: Have your friends made transportation arrangements for when they arrive on Mull?
p.p.s.: Have your friends considered flying from LHR to Glasgow, or have they already planned to do so? If their transAtlantic flight is on a British based carrier, they may be able to tack on the round trip from LHR to GLA at less cost than the train plus the tube. Plus the flight to GLA is only 90 minutes, and they wouldn't have to carry their luggage from LHR to Euston. British Airways is offering some incredibly low round trip airfares on that route during April. Even if their overseas flight is on a non-British based carrier, they may be able to fly north on FlyBe or one of the budget carriers. However, they have to watch out for add-ons, like luggage allowances. And avoid RyanAir! Plus, they don't have to go through Immigration until they get to Glasgow. It's a heck of a lot easier.