That, I am sorry to say, may or may not succeed.
Even if the HOA does not state that a waiver of the HOA of one violation does not mean they give up their rights to enforce the rules for another homeowner, the HOA could still succeed. For example, if the HOA followed Bylaw regulations which allow them to "grandfather" or make any decisions they like for whatever reasons they like, then they will likely be able to have the shed torn down.
However, if the homeowner can show that there is some kind of discrimination, based on race, ethnicity, gender, age, religion, or other protected classification, then the homeowner may be able to succeed because a court (an instrument of state law) will not enforce a discriminatory action, even if it is taken by a private entity.
HOA must have public meetings in MD (you can review the MD HOA Act at
http://www.dsd.state.md.us/comar/Annot_Code_Idx/RealPropIndex.htm#Title%2011B). You can, if you choose, bring this matter up to the HOA.
PLEASE NOTE: Responses here are for information/education only, NOT legal advice and do not form attorney-client relationship! Only licensed attorneys you hire in your state can provide legal advice.