The starting point here is to establish whether the person you wish to provide for would be legally classed as disabled or not, as there are different routes and different advice that would be offered and tailored depending on the precise circumstances.
Often where a person is classed as vulnerable and in receipt of benefits it is important to understand which benefits are funded from where. For instance, a Personal Independence payment (pip) is not means tested but an Employment and Support allowance (esa) is.
Where you leave a gift to someone in your Will, this may affect the benefits they are then entitled to receive particularly where they are means tested. Although any gift may make an initial positive difference to their life it ma not in the long term be enough to meet and pay for all of their needs.
In instances such as this, a discretionary trust is often appropriate and these do not have to be as complex as they may initially sound. Within the discretionary trust which is contained within your Will, you appoint certain people known as Trustees to look after any monies gifted, and also prepare what is known as a Letter of wishes setting out how and when you would want the Trustees to advance/gift any monies to your beneficiary. This is not a requirement but is always extremely useful to guide your Trustees in making decisions which accord with what your own would be.
This type of trust would be protected from any reduction in the beneficiaries benefits particularly where they met the criteria of a disabled person and in particular where they were in receipt of pip. So, their benefit position would not be affected but they could also at the Trustees discretion guided by the Letter of wishes benefit from the monies gifted to them but held by trust.
The precise wording of the trust and Letter of wishes would always need to be carefully considered but particularly where you have an independent Trustee appointed is a very effective means of ensuring that you can gift to who you wish without affecting their long term ability to access an income through benefits which they may also always need.