By changes I am referring to taking away any current food resources it has, drying up any water sources if possible, and closing off access it currently has for moving around. If you can make it hungry it is going to be more likely to have to investigate new food sources you place for it, and this does not necessarily mean the use of baits. It could simply be a baited trap that the rat now chooses to check out more carefully because whatever other food it was happy with is no longer available. Are there pet foods left available for the pets? This is not necessary, and the pets can be fed and the food removed each night, along with any water dishes or buckets. The pets can live without constant food until this rodent is captured. 

So, perhaps start by identifying just what food resources this rat has, and it could be something stored in the garage or foods in the kitchen or something in bags in a closet or cupboard. If you feel the rodent is leaving the house to go outside for food then it brings up the subject of exclusion, and performing exclusion for rats for private homes is a lot easier than it is for mice. Regardless of whatever else you do you need to address exclusion, and get the customer to accept that closing off access into their home for future rodents is absolutely necessary. So much better to fight them on the outside where it is not so sensitive a location. 

If the rat is in a cluttered place it would be very helpful to move everything around, a great step in beginning a process of cleaning up, throwing out, and storing things properly. Rats don't like changes in their happy little home, and making changes stresses them and causes them to move about more. Now you can place a LOT of traps along probably travel routes and have a better chance of intercepting the rodent. Snap traps may be the most efficient, but use lots of them. They are cheaper than replacing gnawed plumbing and electric wires. Bait the traps with something tempting such as peanut butter smeared on a piece of walnut that is tied to the pedal of the trap so that the rodent has to tug on it. Tie the trap to something nearby and check it early each morning. Place traps against vertical surfaces and use pieces of cardboard or wood to direct the rodents to travel directly to the trap. 

Baiting probably should be a last option for dealing with rats inside homes, but may be needed if all else fails. But, baits can lead to dead rats in inaccessible places, and this of course leads to foul odors and blow flies. Trapping should do it, but don't just put a couple of traps in the home - put a dozen or more. Blitz the place to increase the odds that the rat will stumble upon them quickly. Put several snap traps side by side or end to end. Identify travel routes by the presence of fecal droppings or use a UV flashlight to spot urine marks. And, focus heavily on exclusion to keep rats out of the home in the future.