He or she gets an offer saying that if you are rated on a G550, here's your job and you are on salary now. That could be a tempting proposition especially if the type rating cost is tax deductable partially or the salary covers the incurred cost in a short time.
If I wanted to move from the Airbus to a G550 with an established VIP company then, to be honest, if they wanted me to pay for a type rating, I wouldn't even contemplate the employer. Type rating costs are tax deductible to the employer. Add to that that an employer with 'in house' training facilities can offset training costs against aircraft operating costs as they can ensure the 'trainee' has been trained to specific company SOP's.
As captjns has already alluded to, a pilot in that position is not necessarily taking his piloting ability to the new company but his experience as an aircraft operator and, with that many hours, as a Captain. This experience will directly affect the operating costs of the aircraft via reduced insurance premiums based upon the aircraft commander experience. It doesn't have to mean experience on type at this stage in a career, unlike at the start of a pilots career.
There are an awful lot of factors that govern who may do what in the cockpit of large jets. The CAA govern many with regard to heavy crews, rest and who may be relief Captain. The bean counters through the insurance companies dictate what limits may be flown by whom in what aircraft to keep the insurance costs down. If an operator wishes to make money out of RHS ab-initio training then those costs have to be very carefully juggled against the increased insurance risk of conducting training for a pilot who may not then join the company. One prang lasts a lifetime in the memory of the company that has to pay for it.
Rest assured, if the insurance costs start to outweigh the monies paid by the trainees then either the system stops or the trainees pay more. Only time will tell.