It's about 42 miles so 6 or 7 is realistic.
Unless your mate is superhuman I'd say there is no chance of him pulling off the entire route in 12 hours – the record for the running race is something like 15.75! Admittedly these guys are on foot but they are superfit and fully supported. They'll knock in that bad LL pitch in 1/4 of the time anyone with a bike can.
N/S is not the best option either imho – against the prevailing wind direction and you really dont want to be tackling Loch Lomond late in the day. I'd also find the endless gates and fenced fields on the Glasgow approaches a very anticlamitic ending to it all.
The LL stretch is where many folk come unstuck – usually due to lack of fitness and getting the pace skew whiff. If self sufficient and doing the 'route proper' a more realistic target would be 17 with stops.
Also, the FW to Tyndrum pitch is a relatively easy 40 miles. If you are training for doing it in a oner I'd advise some long 60 / 70 miles routes with 12+ hrs in the saddle