How realistic is it that he would come out with money? Is there any risk or losing it (he is convincing me not).
Rule number one of investing in the stock market: never ever spend money you cannot afford to lose.
Because, yes, the money your DP invested could be lost. All of it, most, a little. Profit is not guaranteed.
As for your first question, that depends on where in the bubble your DP bought the shares.
Here's an example:
The shares of company A are today trading at £3. This is below the underlying real value of the business.
One month ago the share was worth £30, a price which reflected the real value of the business. A bubble then drove the price up to £300.
If you bought a share at the height of the bubble, at £300, your nominal loss is £297 (nominal because as long as you hold the shares, you have not realised the loss.) You spent £300 for something that is now worth only £3. You can keep the shares in the hope that it will rise, but as the high price of £300 was caused by a bubble, this is highly unlikely to result in a profit. At best, you can hope to minimise your loss (by for instance selling at £100 if it ever gets that high again, but as the underlying real value of the share is currently only £30 this may take years).
The reason why we can assume it will not rise back to £300 is because a share price is typically based on the underlying value of the business: the real money value of its trade and assets.
A bubble is a stockmarket frenzy which drives the price up not because the company is worth more, but because many people want these shares. This creates a situation where you have too much demand and too little supply. And the price goes up, up, up.
At some point however people stop buying and start selling. Because the underlying value is much lower than the price these shares were traded at during the height of the bubble, the price typically keeps falling until it reflects the real value again.
But not while the bubble bursts. During that time too many people try to sell, this means too much supply and too little demand typically drive the price far below the share's actual value.
If you bought at £30, you have also made a nominal loss, but as this reflected the real value of the business, you have a decent chance of avoiding any loss by waiting for the share price to recover to that point.
If however you now buy a share at £3, you reasonably could expect to make a profit if the share price recovers back to reflect the real value.
(Bear in mind that a huge drop in share price can have a detrimental impact on a business, and recovery is not guaranteed.)
For an overview of what happened with the GameStop shares, the article I link to below tries to give a narrative account of how the stock went up and then down again and what happened to retail investors like your DP (some made a little money, some lost a little. Some made lots of money and some lost a lot).
www.gq-magazine.co.uk/lifestyle/article/gamestop-stock-oral-history
I believe that it is highly unlikely that GameStop shares can be manipulated again to rise as high as they did in January. They are still about 20 times higher than they were before the redditors manipulated the price. (In my view they were undervalued at $3, but I doubt they are worth the over $60 they are sitting at now.)