My fiancé is is/was on 6yrs probation for agg assault-family violence. Recently, picked up another case (Unlawful possession of firearm by a felon) may 22nd. The courts haven't set a court date. The court appointed lawyer advised he takes what DA offers or go to trial. What should I expect, worst case scenario?

Worst case scenario would include his being convicted and sentenced to the maximum possible sentence for the new felony, and in addition having his probation revoked so that he would have to serve the balance of the sentence for the first felony.

Maximum possible sentence would depend on the jurisdiction (which state or is it a federal charge?). And also there may be aggravating factors we don't know about.

Worst case, they turn it over to the Federal Government under the federal law. And he goes away for maybe 15 years.

So his lawyer should know the evidence, how did they find the weapon, where was it, in his house, or was he carry it ? Why did they find it, was another crime happening also ?

But if they have evidence that willl stand up in court, he should in my opinoin do a plea deal, but he should expect to be in prison at BEST, 5 years on this.