Same way how we disable it ;)
For enabling
================================
state = 'STATE_ENABLED'
bigip['LocalLB.PoolMember'].set_monitor_state( [pool_name], [ [ { 'member' => Member.new(node_address, node_port), 'monitor_state' => state } ] ] )
For disabling
=============================
state = 'STATE_DISABLED'
bigip['LocalLB.PoolMember'].set_monitor_state( [pool_name], [ [ { 'member' => Member.new(node_address, node_port), 'monitor_state' => state } ] ] )