3] Tube from Victoria then train from Paddington. This sounds to me the
most circuitous route, but I could be wrong. Possibly the most expensive
too?

If they're close to Victoria, how about the NatEx coach to T123, then
either a connecting coach if there is one or go downstairs and take
the HeX. If you book ahead it's £6 per person. The 9:00 coach is
supposed to get to the central bus station at 9:40 which should leave
plenty of time to go down to the HeX and get to T5 by 10:30.

If they have luggage (keeping in mind that some people still check
bags, particularly inexperienced travellers), this is a lot easier
than the tube since you can put luggage underneath the coach and there
are luggage carts and a lift to make the connection at the bus
station.