Example: 1.If Boris had gone (to the party), then O would still have gone.
2. Therefore, if O had not gone, then B would not have gone.

Analysis: Suppose B wanted to go  , but stayed away ... to avoid O (who has a hopeless crush on him). If this is the case, then the conclusion is false even if the premise is true. O would have gone to the party all the more willingly if B had been there, so the premise is true. Thus, the original claim (1) is true but its contrapositive (2) is false (unlike the contrapositive of a material conditional which is its logical equivalent, i.e., it has the same truth-value).

Conclusion: Transposing (or replacing) a counterfactual-conditional with its contrapositive form does not preserve its truth-value.

How does this differ from the contrapositive of a material conditional, which I present as follows?

3. If B goes, then O goes. 4. If O doesn't go, then B doesn't go.

The bolded implies that 3 is true, yet 4 is false. So a contrapositive of a material conditional fails too? But I just applied the contrapositive, so how could I err? How could 4 NOT be the contrapositive of 3?

To aid remembrance, I change the names of the people to R for Recluse and S for Stalker. Because the original refers to 1 as the premise and 2 as the conclusion, I'll just write 1 and 2.

[Example:] 1. If R had gone (to the party), then S would still have gone.
2. Therefore, if S had not gone, then R would not have gone.

Analysis: Suppose R wanted to go, but stayed away ... to avoid S (who has a hopeless crush on him). If this is the case, then [2] ... is false even if [1] ... is true. S would have gone to the party all the more willingly if R had been there, so [1] ... is true. Thus, ... (1) is true but its contrapositive (2) is false (unlike the contrapositive of a material conditional which is its logical equivalent, i.e., it has the same truth-value).

Conclusion: Transposing (or replacing) a counterfactual-conditional with its contrapositive form does not preserve its truth-value.

I rewrite 1 and 2 above, as 3 (the material conditional equivalent of 1) and 4 (just the contrapositive of 3). The bolded implies the truth of 3. R's desire to avoid S, implies the falsity of 4. So how do 1 and 2 differ from 3 and 4? Besdes grammatical tense, all looks identical to me!

3. If R goes, then S goes.      4. If S doesn't go, then R doesn't go.