An option is just that - an option that he never need to exercise if it is not to his advantage. Assuming they work in the UK more or less like in the US (where I am), typically he will get the option to buy shares in his company at a fixed price - say 50 (options are usually issued "at the money" - with an exercise price (the price he'd pay for the shares) equal to the current stock price).
So if the stock price rises to 60 he can buy the shares for 50, turn around and sell them, and make a profit of 10 (or just hang onto the shares and see if they appreciate further).
But if the stock falls to 40 he does nothing - why pay 50 for the shares when they are only worth 40?
So he cannot lose by signing up. Tax treatment may differ in the UK. so he should consult a tax professional (or his firm may have some documents that explain it). But in the US, the difference between the value of the stock and the exercise price is ordinary income - in the first example above, he'd have ordinary income of 10 (60-50).
Now, the only thing to further consider - is he getting these options in lieu of a higher salary? If so, he'd need to ponder whether the "sure thing" extra salary is worth more than the "speculative options", or vice versa.
But if this has no implications for his salary or other benefits, it's a no-brainer.