Honestly... (and I say this as a leader with a running club, so I'm used to supporting people training for 10k, half marathons and beyond), it would be unwise to attempt to run a half marathon if the furthest you've run is 4 miles. Trying to run more than 3 times further than you've ever run before, on a hilly course, potentially with a niggling injury is never a good idea. Yes - maybe you could walk the second half, but I struggle to see what you'd gain from that (and I often have walk-breaks in halfs/marathons, so I have nothing against walking). Even if you run the first 4-5 miles, 8-9 miles is a LONG way to run/walk (realistically, given the circumstances, will be more of a walk/run). There's a cut off point anyway, which means they may well put you in a sweeper vehicle, or take your number from you. Sometimes they will let you continue on pavements etc at your own risk, but if you're beyond the cut off, then you won't 'finish' anyway.
Has your friend trained for it? You say you don't want to let her down, but if she has trained, then she's going to want to run her own race anyway (she won't want to run/walk if she's geared up to run it). If she hasn't trained, then she shouldn't be running it either. If you're keen to support her, you'd be better off being there at a couple of places en route, cheering her on (seeing a friendly face on the way round makes such a difference).
I think lots of people underestimate the half marathon distance, partly because they see so many people running marathons and this frames the distance as a lot less. Which it is - but it's still hard. Your body needs to be used to running to be able to do it for that long.
I know this sounds negative, but it's more about being realistic - and sensible. If you want to do a half marathon (and in my view, you have to want to do one yourself, not just because your friend wants to do one), then find another one (I'd suggest a nice big one, where there will be lots of people of all speeds and lots of support) and work out a training plan, and train properly. Then you stand a chance of enjoying it and feeling that real sense of acheivement. Be warned - it gets addictive! I signed up for a half marathon when everyone was doing the Ice Bucket challenge (I'd run 10k before, but no further). 3 years on, I'm about to do my 5th marathon. Every time, I say 'never again', but somehow I get sucked in...