The UK housing market has out performed the stock market for the last 30 years.
You will not have financing costs to consider so your rental income should cover your ongoing costs assuming the house you buy has a decent survey and the conveyancing is diligent.
You don't need to leverage the equity to buy another property... this could be your property.
You could invest in a pension scheme but that is unlikely to give you a house to live in when you retire as it is designed to give you an income stream.
You will need both, a house to live in and an income stream.
Suggesting that your should make decisions about securing your future versus risking not being eligible for certain benefits is not in line with how I like to plan my future.
Where will you live when you retire and are no longer working for the family business? Does this business provide you with a secure retirement plan? Those are questions you should be discussing with your partner, regardless of this windfall.
How to best use this windfall, to secure your future, should be discussed in light of whether you want to maximise capital growth or income and how much work you want to do. One of the biggest factors in BTL business is the cost to fund the investment.. ie the interest rates you would pay on the mortgage / loan.. for you, this is zero and you will be exposed to zero risk during to changing interest rates.
You will have ongoing maintenance costs and obligations to your tenants and you are at risk of void periods, non payment of rent and damages by the tenant that are not fully covered by the deposit and / insurance. You are likely to make a profit (because you have no finance costs) and your capital is likely to grow. Taxes are paid on profit and capital growth, so if they go up, so too does your profit / capital growth. It's a good 'problem' to have.
Should you invest in stocks, ISA, savings account, your returns are likely to be lower.
If you were to purchase a property, you would learn how to be a property owner, then when you retire and buy your own property, all of this knowledge and experience will be part of your skill set at a time when you are less inclined to take risks and learn new things.. it will be a lot less stressful.
Does your friend own their own property, have their retirement secured, want the best for you or have their own fears?
Talk to someone neutral, unbiased and informed.