Editing Strava routes without premium

The garmin will follow the route no matter where you start from. Distance to end will obviously not display correctly. You might also need to reload the route when you get to their start point.

I have a similar issue - I'm planning to ride a circular route from a different start point. I tried editing it in a free RidewithGPS account. It's possible, but very fiddly - I had to basically delete a little bit of the end of the route, then move the start of the route backwards, over and over until I had got the start / finish to where I wanted it on the loop. There must be a better way. I'm also not confident that my re-routed section is completely accurate. If you just want to add a section to get from your location onto your friend's route though, this is doable. 

I am probably going to do what Judge dreadful suggests instead - I assume that when I hit the route the Garmin will recognise it and direct me from wherever I join it, even though it's not the designated start point... If not, I have my cobbled together version as a backup.     

Yes. Start the ride from where you're starting it, when the Garmin asks you if you want to navigate to the start point, say no. It will automatically route you from where you're starting.