Apologies, actually re-reading, I think I may have misconstrued and lumped in OP with the other PP whose phone was taken off her. OP doesn't make it clear whether she cheerfully/reluctantly handed over her phone or whether the woman took it from her.
However, I think it's still boundary-crossing to ask somebody to hand over their phone, considering all of the personal stuff that is undoubtedly on there. I feel uncomfortable enough helping elderly relatives and friends who expressly ask me to help them with something, as I'm conscious that I could see something that is personal to them and none of my business - nothing at all dodgy, but a normal message preview popping up from one of their friends to them, not me. I also take random photos of things that may seem odd or even embarrassing to others, out of context, and I'd rather they not see them. Maybe a photo of something in a shop that I'm considering buying, that I wouldn't necessarily want others to know my level of interest in or machinations on!
I also think that, if somebody doesn't know how to leave reviews, it's very likely that in taking/asking for their phone to 'show' them how, you're highly unlikely to end up with a truly independent review, based on their own actual opinions, rather than something very weighted by your own input and presence. Who is going to feel confident typing anything negative, or even neutral, whilst the subject of the review watches them keenly as they type? Thus I believe that my 'domineering and aggressive' accusation does still at least partially stand.
If you really must, there's nothing stopping you from showing them how to do it by leaving a review on your own phone (even just an example one, that you don't submit) and then asking them to think about their own feelings and then leave you an actual review later, if they'd be so kind.
Or just produce a little print-out for them to take away with a few basic instructions or screenshots, if you're so determined to push it (not that there's anything wrong with that); anything to avoid acting disreputably and risking fake/biased reviews, if you DO care about honesty and integrity at all.
Frankly, although glowing reviews probably look better to potential new customers, if it were my business, I would value honest feedback from existing customers so that I could address any concerns and then reply to assure them (and anybody else who reads it) that I have done so - thus hopefully encouraging them to come back again, rather than having been repelled by my heavy-handed insistence on a perfect review (mainly written by me) then and there. You can learn a lot about a business - good or bad - by how they respond to less-than-sparkling reviews.