Unsatisfactory reference
A. The employer can withdraw the job offer if the reference is unsatisfactory and this will not constitute a breach of contract, as long as the offer of the job was conditional on receipt of a satisfactory reference. However, it may still be possible to challenge the withdrawal of the offer.
The employer is entitled to decide on a subjective basis as to whether or not the reference is satisfactory (Wishart v NACAB 1990 IRLR 393). But a potential claim may arise if an employer decides a reference is unsatisfactory on a discriminatory basis: for example, if there are issues relating to a protected characteristic under the Equality Act 2010 or trade union activities.
There is also a possibility that the reason for withdrawal of the offer is something other than the reference. In Halai v Integrated Asian Advice Service UKEAT/0855/03 the reason was found to be the employee requesting equal pay, rather than the reference.
The first step for your member would be to obtain a copy of the reference. They could make a data subject access request to the prospective employer (see Labour Research, August 2017, page 20). The Information Commissioner’s Office provides guidance on this.