What is the process to gaining access to property?

File an "ejectment" action: "ejectment" is "eviction for non-tenants"--for persons in or on the property who are not rent-paying tenants. If your parents can show that their title is good (e.g. that the person or business who sold it to them had good title, could legally sell the property, and did in fact sell to them), they can get a court order directing the sheriff's office or a constable (who does this varies by jurisdiction) to remove this woman. An ejectment action is the only way to remove her--they should file right away and should file on an "emergent" (think: "urgent" or "emergency") basis to get into court and get a resolution more quickly, since they are losing money (e.g. paying for someone else to occupy their property) and are exposed to losses (e.g. the property is at risk of damage) the longer this goes one. Filing on an emergent basis adds to the legal action's procedural complexity, and ejectment actions are slightly complext to begin with: your parents are strongly advised to retain an attorney to help her (even though this is not a landlord-tenant issue, in my experience, most landlord-tenant attorneys would know how to do this). In the lawsuit they can also ask for monetary compensation for costs they incur or damage the woman causes.
You right that the woman claims the property was willed to her. If she makes that claim in court, the court will examine her will and your parent's title/purchase documents and determine who actually does have the right to the property. If it turns out that this woman does have the right to it and it was illegally sold to your parents, they can sue the seller and possibly some of the professionals engaged in the transaction for compensation (e.g. their purchase price, costs of sale, legal fees).