<p>Ethically the person can go with either job A or B. Most jobs today are 'at will' where an employee can leave at any time and the employer can terminate the employee at any time. The term 'loyalty' doesn't really apply. Unless it's in a contract most things like giving 2 weeks notice, etc. are on a best effort basis. In this case the employee hasn't actually started yet but the conditions still apply. To be courteous the person should inform B right away once the decisionis made - not wait until the last second or until the start day.</p>
<p>Practically, if the person now turns down B they need to understand that if A doesn't truly work out, the door to B might be closed - i.e. B may get upset that the person changed their mind and decided to go with another company and might be dis-inclined to hire the person again.</p>
<p>Also, why did A tell the person they wouldn't be employed and suggest B only to turn around a little later and make an offer? It makes me wonder about how solid the offer and likelihood of reasonable security of that position at A is.</p>
<p>I've had someone back out of an offer I gave him after his company counter-offered. Although it meant I now had to look for someone else, I didn't hold any animosity toward the individual since he made IMO a business decision. It may have helped that he told me he wasn't just grubbing for a carrot to get his company to counter and he truly felt bad about the change in decision but he needed to go with what he thought was best for him. I'd actually much rather have him do this than go ahead and come to work with me out of some feeling of obligation only to end up leaving anyway after a few weeks or few months when he truly recived a better offer but after I spent a lot of time and money bringing him up to speed.</p>