Your understanding is wrong. As joint owner he has the right to enter the house regardless of his current living arrangements. Unless OP gets an occupation order, he has the right to live there. He can get a locksmith to break in (but not while OP is at home). Indeed, he can break in without a locksmith provided OP is out at the time. And he can change the locks provided he gives a key to OP. He cannot exclude her from the home just as she cannot exclude him. Having said that, if he does just start turning up at random that could be harassment.
It is unlikely his solicitor has advised him to get a key. Whether he has a key makes no difference in divorce. However, his solicitor may have advised him that he has the right to have a key.
To clear up a couple of other things - as they haven't divorced, there will need to be a financial settlement. The house will go into the pot along with all other assets. If he has bought another property, that will go into the pot too. Whilst the starting point is a 50/50 split, that doesn't mean he will get 50% of the equity in the house. The fact they have been separated for 8 years and, as far as we know, he has a roof over his head is likely to reduce his share of the assets.
Someone was concerned that OP might be liable for his debts. If he has run up significant debts since separating, the courts will treat them as his. They will not make OP liable for his debts. On the contrary, the most likely outcome is that it would increase her share of the assets. To explain how that works, imagine that there are total assets of, say, £300k and he has run up £100k in debts by acting irresponsibly. The courts may take the view that, had he acted responsibly, there would be £400k available. If they decided to split the assets 50/50 (unlikely after 8 years separation), OP would get £200k leaving him with £100k plus his debts.