Its a Euphorbia, sorry don't know which one.
Please tell your B-in-l to be careful if he should break one of the stems. They ooze a milky sap which is quite toxic in contact with your skin and can cause irritation like an acid burn. You need to be particularly careful not to get any in your eye - easily done if you just happen tow wipe a hand across your face after handling the cut stems. Don't let that put you off though - they can be very useful and attractive in the garden. Just wear gloves when moving or cutting them.

Thank you, I'll warn him. It's a lovely looking plant

Agree with the above from Stera. I have to be careful because certain plants trigger eczema on my hands. I think this is the euphorbia that I have in my garden and if it is, it spreads like mad through runners. It is a welcome sight when it flowers at this time of year.

It looks like Euphorbia Martinii. I love it, but it doesn't spread in my garden. I rather wish it would! All the above re the sap is true, although - fortunately - I've never had any personal experience of that.