I think the verse is pretty self-explanatory. It kinda goes along with what Paul said in 1Tim 2:5:
For there is one God and one mediator between God and men, the man Christ Jesus
These verses help establish that Jesus is the ONLY mediator between us and God. NOBODY else can fulfill this position. Not even the greatest Saint in heaven (who would be the Blessed Mother). The ONLY way to the Father is through the Son.
This person is using this verse to try to confuse you into believing that St. Mary cannot “mediate” for you, or assist you in your spiritual journey (because she is not Jesus). Therefore, it is folly for you to seek her prayers.
This is the typical protestant objection to the Communion of Saints in general. Yet, it is no different, in principle, than if you had asked your mother or your friend (or the people on Facebook) to pray for you. Protestants ask people to pray for them all the time. This is a form of mediation, but the prayers are still directed to Jesus. So, while Jesus is the ultimate mediator, many people may petition him for the same intention, on behalf of other people. This does not diminish Jesus’ role as mediator - in fact, it magnifies it.
Protestants believe that the Saints in heaven have no awareness of those still on earth, and are unable to pray on our behalf. They believe the living can pray for the living, but the Saints cannot pray for us because they’re “dead.” Catholics do not believe in the death of the Spirit, and we believe those in heaven are even more closely united with the Body of Christ than anyone on earth, and they are thus able to perceive us and pray on our behalf, just as we on earth can do for each other.
Christians on earth are part of the mystical Body of Christ. It is the SAME Body that those in heaven are joined to (in the most perfect way). There is not one Body for those on earth, and a completely different Body for those in heaven.