The answer is No. First the RESERVE PRICE must have been met, otherwise the sellers are under no obligation to sell or may choose to re-list the property, or negotiate with one or more of the top bidders.
Second if when bidding ends the reserve price has been met, and your client's bid is the highest bid, your client's bid will most likely be selected as the winning bid and you will receive notification about getting your clients to sign and submit the purchase agreement.
But Please Remember: All property listings are sold subject to acceptance of the Bid or Offer by the seller. And sellers generally reserve the right to reject, accept or renegotiate any bid or offer. For Example, if Bidder A and Bidder B both bid $300,000 for a property but since bidder A's bid was submitted first, bidder A's bid takes precedence and is considered the highest bid. The seller however may review both bids and select Bidder B instead of Bidder A as the winning bidder, especially if in the Bid Registration Form Bidder A indicated he will be getting a mortgage loan and needs more time to close, while Bidder B indicates he will be paying cash and can close sooner. Though financing is the responsibility of the buyer.