There are plenty of posts advising against the long term tax and legal implications of overseas working. if there is a very good reason for it and it does not extend then I see no problem.
What worries me is what can be termed 'mission-creep'

As far as I am aware there is nothing legally you need to consider if the employee works within the UK for 6 months or more within a year. In our company, as long as the employee agrees the working hours and the team are happy they can work from a different location. Regarding travel costs, we would not pay for someone to travel back to the UK for a meeting, because it is their choice that they are working from a different country and therefore their cost to bear. but it is something you need to think about. Something else to consider is that you set the right precedent and expectations for your employees. Will everyone have the opportunity to work remotely from another country? What's the maximum amount of time they can work away? What countries can they not work from? (either due to overlap of hours or legal restrictions or risks)

In reply to Gemma:

The main legal issue is the employment and corporation legislation in the country the person is working in. For example, If they consider that the person is a resident for employment law purposes (more likely if they are a citizen of that country), you need to ensure that your contract of employment, pay, benefits etc comply with their employment legislation. This can include a requirement to offer private health insurance in some cases and follow very complex procedures to make changes to the contract/dismiss the individual.

People do sometimes struggle to understand the concept that the other country can decide what rules apply when one of their employees carries out work whilst working in that country. However, we would be the same - if someone was working for an overseas company, whilst resident in the UK in line with our definition of the word, they would have to pay minimum wage etc.

There is no set timescale, after which they would be considered resident in the other country for employment law purposes. Under 3 months is often ok, but no guarantees.

You'd need to make sure you mitigate against it going on for longer, as once it gets to six months it gets messy from the tax side.

The other thing to look at is whether he will be meeting with French clients or customers. That could make it look like you've set up some kind of 'illegal branch' which could crystalise tax and labour law risks much sooner.