The obligatory "question for a friend" is still missing. ;) Otherwise, everything has already been said: Renting out as an investment is nonsense, and while the VFE is high, the interest rate for the new home could be lower, so I wouldn't cling too much to that.
Without numbers and more information, no discussion. Purchase price, equity, salary, current installment, possible rent, existing loan term, interest, repayment, how much is outstanding or paid off, etc. everything that is relevant and comes to your mind.
But you probably already thought of that? Don't the friends have DSL?
Purchase price land: 80000 Euro
Purchase price house turnkey: 300000 Euro
Outdoor facilities: 25000 Euro
Monthly installment for the currently occupied semi-detached house: 1250 Euro
Remaining term of the loan: 18 years
Remaining debt of the semi-detached house: 298000 Euro
Possible cold rental income of the semi-detached house per month: 1300 Euro.
Household net income monthly total: 6200 Euro.
Monthly expenses: 2300 Euro.
These are the figures calculated roughly to get an idea.