The 10-year ban is based on the fact of their accrual of more than 1 year of "unlawful presence" and then departing the US. Even if the government does not know when they left the US, the ban still exists after they leave the US, as a matter of law, even if the government might not be aware of it yet. Leaving the US by land might very well mean that the government is not informed of their departure, but that does not "avoid the ban".
When they next applies for a visa or ESTA, they will be asked about his past stays in the US and whether they have overstayed, so they must disclose their overstay. If they lie, that is fraud and a much more serious violation. If the officer sees that they did not have a departure record the last time, the burden of proof will be on your friend to prove that he left when he says he left.
If they wish to enter the US during the duration of the ban, they would need to apply for a waiver. If they wish to visit as a nonimmigrant, and their visa is denied due solely to the ban and not due to immigrant intent, they can apply for a nonimmigrant waiver. However, they are almost certainly going to be denied also for immigrant intent due to the long overstay, and there is no waiver for that.
If they wish to immigrate, they would need an immigrant waiver, and an immigrant waiver for this ban is only possible if they can show that their US citizen or permanent resident spouse or parent would suffer "extreme hardship" if they can't be in the US, and that is extremely hard to prove, especially if the US citizen spouse has joined them in Europe, so isn't separated from them.
If they are married to, or will be married to, a US citizen, why not just get a green card within the US via Adjustment of Status? As long as they haven't left the US, they can do Adjustment of Status as the spouse of a US citizen without any waivers, regardless of how many years they have overstayed. Or do you mean that they don't want to stay in the US?
