This is a good idea. Although I’m not sure a Lent tab/category is the best way of going about it.
I think a better implementation would be to use Bill Splitting:
Monzo build Bill Splitting for outgoing Monzo-to-Monzo payments. You could then send the payment and Bill Split the whole amount to the person you sent the money to.
The request you sent now appears in the Bill Splitting requests section of the app in the Payments tab.
NB: since you can already Bill Split outgoing bank transfers, you could do this if you got someone’s Monzo account number and sort code, but that’s definitely not as easy as using a Monzo-to-Monzo payment.
A possible implementation for this:
Monzo build the ability to request money from another Monzo user in-app.
(ie: you send a request to Person A for the money. Person A gets a notification and a request feed, like Bill Splitting, which they can accept.)
You decide the amounts they’ll you back in and then request the money in those amounts.
(eg: request the £200 you lent as four requests of £50.)