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Yes I agree with you. It is better to invest only £50 in bitcoins rather than reinvesting them in premium bonds. Because with this small amount of investment he won't be able to achieve much but if he wins a bigger prize then he can suddenly become rich from here. There are many chances to achieve big things because he has nice luck. If he invests them in Bitcoin then he can get maximum double profit in one year because now Bitcoin is in deep dip. But if he invests in bonds he can win millions of dollars if he is very lucky so it will be better for him.
I didn't say he should put that £50 in bitcoins. I said premium bonds are not necessarily the best investment considering there are better-paying alternatives, such as fixed-term saving accounts or ISAs.
Investing in BTC is a completely different category, as it's a high-risk investment.
I do understand the sentiment of putting small amounts into high-risk investments, hoping for better gains, but I tend to agree with the opinion that it's best to adopt a sensible approach to investing and act the same disregarding the amount you have available. If you don't keep gambling smaller amounts away, then they have the potential to slowly cumulate into much higher figure (with a help of compound interest etc).