If you have read the chatlogs (leaked messages), Matt mentioned Liz has a company benefit and that their Audit team has listed her for benefits removal, but he can remove her from that list if she do what he says. My understanding is that this benefit is for her sick mother which entitles her to use a nursing home. Liz doesn't care if Matt expose the pic and said MC will still believe her, but she can't afford loosing her mother's benefits. Thus, the blackmailing continues.