There is nothing wrong in sending large amounts of money, but as
@Samatlewiss said, you'd need to do it in small batches or the money might disappear into a void.
There are two ways to send money:
1. Wire the money directly from your bank, into their bank.
You can only send a maximum of £999,999 at a time. But you can do this multiple times.
It's always worth sending £1 first, just to make sure they receive it. Note that wiring money incurs a fee. You'd lose £66,667 every time you transfer £999,999 to some one. (They'd receive £999,999, but it'd cost you £1,066,665 to send it.
Also note!!! The person you are sending money to, resets every time you send money. You can easily send money accidentally to the top player in the list if you don't check this.
2. Give money directly to the person.
For this, you pull the money out of your bank, again, you can only withdraw £999,999 at a time, but you can do this multiple times.
You then hold say, £10,000,000 on your character. Then as before, use the Y menu to give your friend £1, make sure he receieves it, then send over the money to him in batches of £999,999. Check each time you send some over that he has received it. Once he has the money, he can then store it in the bank.