Welcome to the forum, Vicky!
I’d offer your partner three comments/suggestions.
1 - it is hard - it’s meant to be hard - because that’s what gets the results. So it’s normal to feel that it’s hard to produce the sentences in time - everyone feels that at first. When the sentences get too long, just do what you can, and listen to the rest - it’s all valuable exposure to the patterns, and the method doesn’t depend on you getting everything ‘right’.
2 - he’s not meant to be able to remember it all at first. That’s a hangover from school approaches to learning, where if you can’t remember something, it means you haven’t learnt it, and you get a bad mark. That’s not a good reflection of how learning actually happens - the more exposure you have to something, the more often you try to recall it (successfully or not), the closer you are to a reliable, active memory. If he keeps on pushing on (through 5 or 10 sessions, maybe) and then comes back to the first 2 or 3 that he’s currently finding difficult, he’ll discover they’ve become easier.
3 - if he wants to see what it’s like when someone really, really finds it tough, but commits (over time) to the process, he could do no better than having a look at Tatjana’s journey (even if he skims it rather than reading every post!):