The partner may be a partner in name only but is an independent contractor to the firm who receives a 1099 misc. Law firms designate an IC with clients as a partner to boost the attorneys profile with clients but the partner will be paid as an IC based on his/her own billings. Some law firms designate employees as partners but are paid as w-2 employees.
If the partner is an IC for tax purposes then that person can establish a solo 401k plan.