Reunion for 90 days

Can anyone advise me on this my partner is meeting me in la reunion for 90 days and he is going by ship but the date of the ship only goes up to June 18th and he would need to leave on the 23 aug  I will be paying. For studio in dodo spot

What happens at Immigrations when he has a return ticket but the invitation letter says 23 aug he leaves and do I have to put the full amount on his account or can we buy travellers cheques for 18 June until 23 aug  I'm finding it difficult to find this info .

I fly on 21 may as my 6 months is up so reunion then seychelles but Seychelles don't need all this paper work it's cool there !