We bought our house in using an SCI (and by an accident of fate I have also managed a few SCIs for clients). So I can give you some basic answers.
Money
Best way to set it up is so the money to "buy" the house is loaned to the SCI by each party in their country of origin. That way there is no tax liability in or UK for your friend - assuming he is simply using his "UK income tax paid" savings to fund it.
So your friend sets up a UK loan agreement between him personally and the SCI. He can either do the loan as interest free or interest bearing - the cleanest tax way is to make it interest bearing at a commercial rate (eg Bank Base Rate).
Ours is interest bearing, but with no repayments until the end (you can pick as long as you like up to 99 years).
The down side is he will then potentially pay imputed income tax on the interest. To give a round figure example: lend 100,000 at 5% a year, means potentially paying at basic rate income tax, 20% of that interest = 1,000.
As mentioned, your friend can lend it interest free. But depending on your friend's relationship with HMRC, this could lead to them charging imputed tax anyway. Same might happen if you use a non commercial interest rate (eg 1%).
Now of course, you need to lend money too. This will be the proceeds of the house "sale", when you transfer it to the SCI.
(Note: It MAY be possible for your to "transfer in" the property at a value in lieu of money, but you really need the Notaire for that answer)
Buying the property
It MAY also be possible for the SCI to buy part of the property, rather than all of it. But that's another Q for the Notaire (and I wouldn't).
Cleanest way is to then sell the property to the SCI. You can probably short circuit some of the costs by not needing an agent and so on, but this of course triggers some tax liabilities for you and some property transfer taxes.
You don't need to put cash in first if you pledge the house proceeds - as long as your friend can stump up the fees etc.
At the end, through the SCI, you and your friend own the house at whatever percentage split you agreed when setting up the SCI. (50/50 is best!)
Renting the property
SCI's can only rent out unfurnished property. So you then need a rental agreement to yourself, which can be on whatever you decide (and which are legal under French law). It is this rental agreement that helps with the rights to remain in the property.
What happens on death?
Both of you need to make/update wills and mention what happens if you die. You each leave your shares in the SCI to... someone/some people. You also leave the rights to reclaim the loan to that same someone/some people.
So if your friend dies first - and he has left the shares/loan to his son - then son inherits 50% of the SCI. But he is bound by the on the loan agreement - so he can't ask for the money back OR to sell the house without majority agreement.
If you die first - and you've left you half to your daughter - same applies. Neither party can sell until both parties agree.
There's more but I've run out of time to post this!
Kind regards
Ian