It can be done a couple of ways.

I don't know which specific French brokers offer which features, but there are general categories.

Most basic stock brokers in France will, I believe, want/require you to hold cash only in Euros, and by extension require you to buy only things listed in Europe. This is the easiest type of account to open. The Euro thing isn't a show stopper in this particular case as Berkshire is listed and trades in Euros on IBIS, the Deutsche Bourse electronic exchange. You can buy and sell the shares in Euros without any conversion to or from dollars. But this approach is pretty limiting over time. I would expect high fees, many restrictions, bad service, and hard sell on terrible high fee products. This can be a simpler "starter" account, perhaps at the person's bank, just don't forget to reevaluate the choice again.

In the middle ground are higher end brokers, generally add-ons to a "wealth management" bank account. High fees, but a full range of products, and accounts in multiple currencies. But very high minimum account size.

I use Interactive Brokers. They allow accounts by French residents and citizens. Frenchresidents are legally supported out of IB's subsidiary in Ireland, but it's basically all the same thing. Opening an account at IB can be a few more steps, there is a minimum commission/data charge of about $10 a month, and their interface is a bit daunting the first couple of hours. But the commissions are too small to bother tracking, and they plug you directly into the global forex market so you never pay a currency spread again. They will generally let you trade any product, including bonds and funds, that it is legal for you to own based on where you live. This is an excellent longer term choice for when you know what you want in the way of capital allocation.

Just a reminder, it is forbidden for EU residents to own US mutual funds or ETFs. Nobody asked, but this catches people by surprise sometimes. (as Monaco is not in the EU, this rule does not affect me)

Jim