Her friend has no idea what he is talking about. She is entitled to appoint whoever she likes, and the only possible legal challenge to an executor can be after death if the executor is not doing their job properly.
In a situation like yours, it would be perfectly normal to appoint a relative as executor. My mother is much the same - she has left us everything equally and appointed my sister and me as joint executors. This is perfectly sensible and in fact the best way- then it's up to us to sort it out between ourselves when she's gone. She knows we get on well and will do it right. And I wrote the will because it's a very simple one. As in your case - "I give everything I own to my son Robbert and appoint him to be my executor" is the simplest it can be, that's all it needs to say, no fancy legal wording could possibly make that any better, and all you have to do is make sure it's signed and witnessed correctly according to the law of where you are. The most sensible thing is to make you the executor. My Dad's wasn't much more complicated, I wrote that too, and it got through probate with no problems.
I've never heard of a will being written on a lawyer's letterheaded notepaper! What a lawyer is most likely to do is prepare it on stiff legal plain paper.
Unfortunately the rest is less good news. If the debts are greater than what you can sell all her property for, we have an insolvent estate and in the same situation, I would get a lawyer to help me do the job of executor. Everything will have to be sold to pay the debts, including the house, which means nothing left for you, but on the other hand you don't have to pay out of your own money. Medicaid will have to suck on air for the rest. Just like a normal bankruptcy, in fact.
So just a few thoughts but of course I don't know where you are so I don't know the local law about property registration and ownership. So check these through with a lawyer to see if they might work.
It might help to transfer the house into both your names and that would make it more complicated to get you out. I only say might because where I am, the law of equity would say "you didn't actually pay anything towards the house so the value of the house all belongs to the deceased".
Which brings me to the next idea - where I am, if you don't own any of the house but live in it, you can register a caution on the land register. This makes it impossible to sell unless you agree to have the caution removed. This goes back to the days when the husband worked and earned all the money so the house was all his according to equity because he'd paid for it all, the wife stayed at home and kept house, and very often the house was only in the husband's name. Very precarious position for the wife - she could easily become homeless at her husband's whim. But what she CAN do is register a caution, she can do that without telling her husband, then her husband can't sell it without her consent, and she's safe.
Talking of husband and wife brings me to the last idea, which I've talked about with my sister and her husband when I wrote their wills. Where I am, there are two ways of owning a house jointly, joint tenants or tenants in common. Tenants in common each own their proportion of the house and can leave it by will. This would be most appropriate for roommates buying somewhere together. But joint tenants own the whole house together, and if one dies, the other joint tenant(s) automatically inherit(s) the deceased's share, whatever any will says. So as I've said to them, you bought as joint tenants, didn't you? Then whoever goes first, the other is assured of somewhere to live. Basically they got married because it guarantees no death taxes on the house. (That was a big reason for wanting gay marriage - personally as a gay man I don't care two hoots about marriage, but the tax breaks are worth having.)
If you and your mother were joint tenants and that's how the law works where you are, that would be ideal. When your mother passes on, the house automatically becomes yours and Medicaid can't touch it. This is English law and you're obviously American because you mentioned Medicaid, but if the concept of joint tenancy applies in your state, that would be the way to go. Consult your tame lawyer again and if this would work, get him or her to draw up a transfer/conveyance/whatever it's called for Mom to sign that would put the house in both your names as joint tenants.