For convenience and/or avoiding some banking fees, one may want to transit a friend's money through one's account. E.g. Alice's bank account in France -> Bob's bank account in France -> Bob's bank account in the US -> Alice's bank account in the US.
Can that cause any issue (e.g., triggering some tax event)?
Assumptions:
Assuming that Alice and Bob are US tax residents, the obvious question is whether Alice made a gift to Bob when she transferred the money to Bob, and Bob made a gift to Alice when he returned the money, and if so, is the amount large enough that gift-tax returns might be required to be filed. No gift tax might need to be paid if both parties choose to charge the amount to their lifetime combined estate and gift tax exemptions (currently $5.4M or so), but with transactions of six-digit amounts (anywhere from $100K to nearly $1M) as mentioned by the OP, these reductions can soon deplete the exclusion, especially if repeated at frequent intervals.
If the money is regarded as a loan from Alice to Bob that is being repaid in the US, it is best to have documentation to say this is a loan with interest at rates at least as large as the minimum specified by the IRS, and even if the loan document insists that the loan is at zero interest, Alice still has imputed income (as if she did get interest from Bob at the IRS-specified minimum rate) that she must declare on her income tax return.
