I had a look at a gearbox in at Les's today that had the cases off.
After a bit of a conflab with Les, we determined that some when you've selected 1st, the selector finger has then moved out of the 1st/2nd rails (who knows how or why)
& you've unwittingly selected the 5th/Rev rail. Hence you've selected 2 gear at once which makes the car stall.
Something is going on with the selector finger & how it runs in the selector rails.
But what I've just typed is all anecdotal anyway...
You are still going to pull the box, take it to your mates dad, who still won't find anything, put it back together & give it back to you even though the problem hasn't been found.
You'll then be even MORE annoyed that you've spent MORE time & money in not finding the problem, when you should have just taken it to Les to repair in the first place!!!