Is it halal to advice a firm that is intending on using leverage for a buyout?

On the buy side, there are a few PE companies that are evergreen and buy equities in a company with equity. So if I work in such a PE firm, when I’m working with the executives of this company on how much loan to get and on which interest rate and and, how can this be purely halal? Don’t you think this enters a Grey area? 

I agree that even as advisory only you should try to specialise in a field that is not advising haram activities as much as you can. That is possible. If your conscience is upset with something that is usually a good sign. However, the general principle still applies: advisory-only services, even to a non-sharia-compliant business, are at heart permissible. Not holding this principle would lead to an intolerably difficult job market for Muslims.