Close to zero chance to make this connection, their only chance is if the bags are interlined (tagged to SLC from BKK) as this would allow dropping the bags at TBIT after customs rather than having to check bags at United. UA has no obligation to put them on a flight the next day if they arrive late -- this is the difference with separate tickets, you take the risk of delays (same ticket costs more because the airline takes the risk and pays for missed connections). Worst case they have to buy new tickets the next day; best case UA puts them on stand-by (but note that first flight each morning tends to be sold out at LAX, so it is likely to be an afternoon flight the next day). Flights from BKK tend to have longer customs checks than some other arrivals (as they tend to contain more contraband, so they are profiled for greater inspections). Combined with today's terrorist attempt against a US airline -- causing more security checks for the next month or 2 -- it is a sure thing that 90 min connection at LAX will not work for TBIT to UA (they are at opposite ends of the airport).